Why I don't believe gangs can loot enough to survive
Let's say a gang goes house to house robbing from each apartment owner. They're liable to knock down doors with home-made battering rams (similar to SWAT teams), and merely brandish arms, for to shoot means possible death on both sides and loss of ammunition. Easy pickins' to steal; risky to shoot at close quarters and penetration.
As they accumulate wealth (an abundance of supplies in this case) and valuables, then they'll get loaded down by them. They have to either a)hole up in one of the other apartments, or b)have a wheelman and a vehicle in the form of a truck to haul their loot back to a headquarters.
The former means it's probably not any more defensible than the other apartments, it's just that they have more weapons. This means at some point, when they sleep or are high (both very likely), others will see their wealth and steal from them, because they concentrated all of it in one location.
The latter means transporting it down to the vehicle, at which point it'll probably be stolen as it's being moved to the street, or else as it accumulates at their headquarters, then it's all concentrated at that place....a lot of wealth. Now unless they know strategy and have a bunker with machine guns, it'll be taken, either from any police or soldiers left standing, or another gang since they have the loot they need.
In that case, a large portion of them will be annihilated by the other on either side and huge amounts of ammunition will be used, but tear gas and flash bang grenades are more likely since both sides wish to not waste ammunition. It may be easier to simple burn them out, for there's no rules anymore. It's a loss of supplies, but removing the threat is probably better. It's that or a siege just like medieval times, and this means manning it 24/7.
All of that is contingent upon enough firearms, ammunition, affiliated gang members, a working vehicle, and fuel sources. That's a tall order.
Most won't know how to source water other than stealing it. Even if they have a great location up high such that they can maintain lookouts, then they'll have sewage issues, won't they?They'll be forced to leave. Even at ground level, that will become a major issue, and people will get sick quick, just like they have in the history of warfare in conflict zones.
Highly organized crime families are a different breed altogether. They could survive since they have power structure, connections, more arms, supplies, but they don't have survival skills. They will run into the same issues of clean water and sanitation. In both cases of organized crime or gangs, there's liable to be a coup d'etat to knock out the current boss. Maybe a series of them as conditions deteriorate.
Actually, I think a lot of gang members will run, for they'll see the handwriting on the wall, and they'll either think a) start my own thing b) get the heck out of Dodge.
The only way I can see sociopaths making it, will be if they locate and seize supplies early from a warehouse or series or warehouses. These are most likely to be commandeered by local governments and law enforcement/soldiers, aren't they?
Is there something I'm not seeing?
Remember, the only reason criminal win, is that law enforcement can't use the same tactics of essentially no rules. If law enforcement can literally have no rules, the criminals can't last. The only way they could would be a loss of law enforcement due to collapse, leading to martial law enforced by soldiers.